Idle Circuit Questions

[David Jones]

Hi Cliff.
David, from over at Performance Years here. I just discovered your forum. Great service!
And yes, I have a question..........or questions?

Right after I got your book (about 2 years ago?) I took a carb I found at the local pick and pull and dove in to build myself a Qjet. The core I used is from an Olds Toronado (17059251 from 79?) and I feel like I've learned a lot about the Qjet from the experience. I've built a couple since then and helped a few locals out with theirs. This weekend due to the rain, I had a few minutes (all day...) so I thought I'd pull mine down and rethink the idle circuit because it smells a little rich at idle and because....well....I wanted to:). I have the idle tubes at .037". Idle bypass air (added to the carb at the build time) at .082. Idle mixture screw hole opened up to .089. Idle down channel restriction at .055. I plugged the upper idle air bleeds in the main body and relocated them into the air horn and they are .052. (why did I move them? Something I read by GTO Karl led me to do it. Because, more than anything). Also when I built the carb, I got rid of the choke altogether. I haven't missed it or the fast idle stuff. And the carb looks great to boot

In order to get the idle speed I need I have the primary blades open some and I'm considering opening up the idle bypass air some to close the blades a bit. I was thinking to the next drill bit size in my drill bit kit to .085. Bigger than that maybe?

While I had my carb apart Saturday I drilled the mixture screw hole out from the .089 and opened it up to .096. I think I went to far? With my .037" idle tubes I don't start hurting the idle stability until the screws are nearly seated now (after the size increase). At the .089 previous size  they had effect at about 3 turns from lightly seated. Would I gain some control back if I put in slighty smaller idle tubes? .036"  .035"

My setup

1969 Pontiac GTO, 406CI. TRW pistons (6626...2262 forget the number) stock rods and crank, factory intake with the ex crossover removed, the water crossover divorced and ported to RA IV gaskets. No carb spacer. 1 3/4 Doug's headers, #48 heads mildly worked by Ken Keefer, Turbo 400, 3.55 gear
The cam is a Comp Cams Hyd Roller........Straight up.
Gross valve lift .513 intake and .528 exhaust. That's with 1.50 rockers. Mine are Harland Sharp
Duration @ .006 tappet lift 276 intake 280 exhaust
valve timing @ .050 intake 2btdc open / 38abdc close
exhaust 48bbdc open / 4- atdc

duration @ .050 220 intake 224 exhaust
lobe lift .342 intake .352 exhaust
lobe separation 112.0

Any other suggestions, other than quit messing with stuff? 

[Cliff Ruggles]

David, the rich idle problem is from moving the upper idle airbleeds into the airhorn.  The .052" bleeds, .037" idle tubes and .055" idle downchannels are simply too "generous" for your application.

I would open up the upper idle airbleeds to .055".  This can be down with some grease on the drill bit to catch any metal chips without removing the carburetor from the engine.  Test run it afterward, and if it responds well, you may even want to go to .059".

Don't worry about going too large, as the casting can be tapped for 4-40 screw in brass removable airbleeds if it doesn't work as planned.

Personally, I don't like moving the upper idle airbleeds to the airhorn, taking them from "indirect" to "direct" airbleeds.  They have a more significant impact on the main fuel delivery system when we do this.  The factory stopped doing this in 1975 for a reason, and eventually went to the single main airbleed design across the board around 1980.  They also went to the "M" style primary metering rods with .036" tips, in lieu of using the "K, L, or P" rods with .026" tips.  The factory does everything for a reason, and I would imagine they were able to maintain closer control of the A/F ratio across the speed/load range with ever tightening emission standards......Cliff

[von]

  I wonder if my rich idle problem could be a similar air bleed situation? After reading your excellent book I set my carb up close to your second recipe specs. It's 7043202 ('73 Chevy 350) on a 396 (404) with stock intake, headers, 10.0-1 CR, and 221/233 dur @.050 lift cam. My upper idle air bleeds are .077 and lowers .062 (both unchanged), idle tubes drilled to .038, idle down channels .052 (unchanged), idle bypass air is .109 (unchanged), and mixture screw holes were drilled to .094. I have to turn the mixture screws in to about 1/2 turn before idle slows any and vacuum decreases. Turning them out does nothing. If I pull a a manifold vacuum hose off, idle speeds up and smooths out from the lope it has. I've verified the power piston spring is allowing vacuum to keep the primary metering rods seated at idle. This carb has approx .120 main air bleeds (unchanged) and I have #75 jets with 44B rods in it. It has 13" of vacuum at idle (manual trans).

[Cliff Ruggles]

What happens if you seat the idle mixtures screws?

I've never to date been happy with any of those carbs that have the .120" main airbleeds.  You can't get enough jet in those for a good clean fuel curve.....Cliff

[von]

If the mixture screws are seated, the idle slows down but not enough to kill the engine. the left (driver) side more than the right. What bothers me the most is that the pulling off of a vacuum line speeds up the idle so much. Is that indicative of too rich an idle mixture and/or not enough bypass air? Are you saying  the .120 main air bleeds do have an effect on idle fuel curve?

[Cliff Ruggles]

Being able to slow the engine shows some control, which is good.

I've never liked the .120" main bleeds.  I get more of those units in here than any other for custom tuning.  Just too much emulsion air, requiring too much main jet and not enough metering rod.  Even going that route the fuel curve goes lean.  The smaller airbleed set-up is a better deal, all the way around.....Cliff